Because the primary endpoint of the study is change in 25(OH)D from baseline to delivery, the primary analysis will be restricted to participants who have remained in the study until delivery and provided a blood sample within 6 weeks prior to delivery, at delivery, or at the post-delivery visit (completers-only analysis). Typically, multiple imputations would be used to impute missing values in support of the favored intention-to-treat analytic approach. Because the multiple imputation model for this analysis would have required variables also measured in the final blood sample, however, it cannot be used to impute cases with a missing final blood sample. Thus, to assess the primary findings' robustness to assumptions about the missing data, we will perform a sensitivity analysis. Missing data will be imputed under the following conditions: cases with missing endpoints that experienced no change in both groups; experienced the group-specific median change observed in completers; experienced no change in the 400 IU group and minimal change in the 4000 IU group. In the secondary analyses using multi-level mixed-effects models for longitudinal modeling, all available data points will be used, as it is not necessary to delete cases with missed timepoints when using this approach.